By gently pulling a metallic contact apart a conducting
nanowire can be formed. During the last stages of the pulling a
neck-shaped wire connects the two electrodes, which is reduced
to a single atom upon further stretching.The complexity
of the atomic structure of contacts is removed if the weakest
point is reduced to just a single atom. The properties of the
contact are then dominantly determined by the nature of this
atom. Atomic contacts have thus proven to form a rich test-bed
for concepts from mesoscopic physics. Properties investigated
include multiple Andreev reflection, shot noise, conductance
quantization, and conductance fluctuations. In addition,
pronounced quantum effects show up in the mechanical properties
of the contacts, as seen in the force and cohesion energy of
the nanowires. More recently, similar techniques are being used
to contact and study individual organic molecules.